Ink jet printing processes are essentially of two types:
In continuous stream ink jet printing systems, a recording liquid is emitted in a continuous stream under pressure through a nozzle. The stream breaks up into individual droplets at a certain distance from the nozzle. If a specific location on the recording sheet has to be printed the individual droplets are directed to the recording sheet, otherwise they are directed to a collecting vessel. This is done for example by charging unnecessary droplets in accordance with digital data signals and passing them through an electrostatic field which adjusts the trajectory of these droplets in order to direct them to the collecting vessel. The inverse procedure may also be used wherein uncharged droplets end up in the collecting vessel.
In the non-continuous process, or the so-called “drop-on-demand” systems, droplets are generated in accordance with digital data signals only if a specific location on the recording sheet has to be printed.
The speed of modern ink jet printers has to increase steadily increasing for economic reasons. Digital images, captured with digital cameras or generated by scanning of silver halide camera films, may be printed with these modern printers at a quality level that makes these images nearly indistinguishable from classical copies on silver halide materials. Images produced in this way need to have excellent storage stability even under adverse conditions. This can only be achieved by using a finely tuned system of recording liquids (respectively the dyes contained therein) together with a suitable recording sheet.
Recording sheets suitable for these printers need to absorb the recording liquids very rapidly, in particular during the printing of photo realistic images. Recording sheets particularly suitable for this purpose comprise nanoporous inorganic compounds, preferably oxides, such as aluminum oxides or silicon dioxide, or oxide/hydroxides, such as aluminum oxide/hydroxides. These recording sheets are known as “nanoporous” recording sheets.
Nanoporous recording sheets absorb the recording liquids very rapidly (in the microsecond range) by the action of the capillary forces of the nanoporous compounds. Polymer based recording sheets absorb the recording liquids more slowly (in the millisecond range) by swelling of the polymer.
Most of the commercially available combinations of recording liquids and nanoporous recording sheets do not satisfy all the necessary requirements. The black dyes used nowadays do not have all required properties, such as very high brilliance (saturation), a suitable hue, good light stability, good resistance against degradation by ozone, an excellent diffusion fastness. They have to penetrate into the recording sheet and should not show dye aggregation on the surface of the recording sheet (“bronzing”) and they need to have an excellent solubility in the mainly aqueous recording liquid.
C.I. Direct Black 168 of formula (I)
is a widely used, commercially available black dye.
Recording fluids for ink jet printing containing C.I. Direct Black 168 are described in patent applications WO93/24,330, EP 0,885,940 and US 2002/0,121,219. However, if used for dyeing paper or in recording fluids for ink jet printing, it is not soluble enough and it gives recorded images having insufficient water-durability, showing bronzing and a non-neutral hue on nanoporous media. It causes clogging of nozzles of the printing head due to deposition of dye. Direct Black 168 has to be used in a mixture with pacified C.I. Reactive Black 31 in order to provide blacks with a neutral hue.
C.I. Reactive Black 5 of formula (II)
is also a widely used, commercially available black dye.
Recording fluids for ink jet printing containing C.I. Reactive Black 5 are described in U.S. Pat. Nos. 4,257,770, 5,358,558, 5,935,309 and in patent application US 2006/0,174,800. The tint of blacks in recorded images, however, is more bluish than black.
C.I. Acid Black 1 of formula (III)
is also a widely used, commercially available black dye.
Recording fluids for ink jet printing containing C.I. Acid Black 1 are described in U.S. Pat. No. 4,365,998 and 4,371,582.
The light fastness of this dye, however, is very poor on plain paper and on polymer based recording sheets for ink jet printing. The light stability on porous media is somewhat better, however, it is still far away from the request of today's market. This dye also has a bluish hue and cannot be used alone as a black dye in recording liquids for ink jet printing.
The black dye of formula (IV)
is described in patent application US 2007/0,050,926 as being particularly suitable as a light-fast component in a black recording fluid for ink jet printing. However, its resistance against degradation by ozone is bad.
There is therefore an urgent need to provide novel, highly water-soluble black dyes with a neutral hue, excellent light stability, excellent water fastness, no bleeding, excellent resistance against degradation by ozone and a neutral hue even after degradation by light or ozone.